I use overlays in resource packs so that I can not make a mess in the main directory "assets" i.e. I name the directory for example. "1-21-4-overlay". Regarding this, in the changelog there is a note about moving slots to "gui/sprites/container/slot" from "items". One sprite does not seem to work with the resource pack, I will only share part of the resource pack to better illustrate the problem. It only affects the "smithing_template_netherite" sprite, which instead of using a different look, uses from the default look as it is in vanilla Minecraft. In general, the resource pack I am uploading is based on 1.21.3 and has the included changes before, as well as the changes after application saved in the "1-21-4-overlay" folder, but has the overlay saved to the latest snapshot